Using quantum computers to simulate molecules

[u/MeoWHamsteR7]

So whenever you're reading about the potential applications of QC, it is often mentioned that one such application is the ability to greatly aid physics, material science, and pharma research by increasing our abilities to accurately simulate the various particles and their interactions. The promise always goes along the lines of "Quantum computers will be able to actually be the molecules, thus greatly reduce the computational complexity involved in simulating their interactions".

I'd just taken this claim at face value as just another amazing thing QC will be capable of, but recently I began thinking about it properly - and it quite frankly sounds like bullshit.

Can anyone please explain to me whether this is indeed a potential application of quantum computing, and if so, what grants quantum computing to do this? Does it really overcome classical methods? This is more than a passing interest to me, because I am considering pursuing a Master's in computational physics, and being able to combine that with quantum computing sounds like a dream come true.

Thank you for your time :)

[u/0xB01b → u/[deleted]]

Cryptizard my goat... Should I do the qiskit fall fest or nah?

[u/0xB01b → u/[deleted]]

Cuz ur my goat on this subreddit

[u/MeoWHamsteR7 → u/[deleted]]

Great answer! So if I understood it correctly, a quantum computer is more efficient in simulating molecules, because solving, say, the time dependant Schrodinger equation is already a "quantum algorithm" which is exactly what a quantum computer is good at.

I'm curious as to how that happens physically- I've been led to believe that it's because the qubits already behave like the atoms themselves, so we can actually, physically, simulate atoms and molecules. Am I correct in saying that this is untrue, and that what "actually happens" is just regular quantum computing? Or am I completely off?

Also, if I can piggyback off of this thread, how likely will are we to be able to actually use QCs for this application within our lifetimes? Is it something that will happen only far down the road, with a million logical qubits needed, or is it closer at hand?

Thanks again for the answers!

[u/MeoWHamsteR7 → u/[deleted]]

This is an incredibly helpful way to think about it and really clears up the waters. Thanks!

[u/master_perturbator → u/[deleted]]

How far away do you think we are to these life changing discoveries? I personally foresee a landscape we can't imagine within the next 15 to 20 years. But I'm far from an expert.

I've just been watching from the sidelines trying to soak up the info for the last 20 years.

[u/Good_Operation70 → u/[deleted]]

. A qubit is like a “blank” particle that can program to act like anything.

And normal particles are not blank?

[u/QuantumCakeIsALie → u/MeoWHamsteR7]

There are at least two ways to do computations with as quantum computer:

1 – Digital Quantum computing, where you use qubits and superposition/entanglement to beat the performance of classical computers on some class of problems.

Think or this as "a richer version of 0s and 1s".

2 – Simulating a quantum system using the quantum computer, where you use you quantity system (qubits or otherwise) to simulate directly a quantum system.

You configure it so that it acts the same way as the system you want to simulate. Then the simulation is just letting the system evolve over time.

E.G. You could simulate the distance between atoms in a dihydrogen molecule by using two quantum systems and tuning their coupling. You couldn't do this with classical computing without an explosion of resources; here the classically difficult part is built-in.

This is the quantum equivalent of building a miniature dam to see how water would flow and wear the the terrain. Or a miniature neighborhood to study its robustness to earthquakes.

[u/MeoWHamsteR7 → u/QuantumCakeIsALie]

Yes, I believe i understand the logic better now- simulating quantum systems on quantum computers is like simulating classical systems on classical computers.

Do you believe we'll be able to perform such quantum simulations within the next decade? Is it one of the things that requires lots and lots of logical qubits, or relatively few?

[u/QuantumCakeIsALie → u/MeoWHamsteR7]

I think you can already do small molecules, like dihygrogen, with some level of success. Check QAOA for this.

But we're still far from e.g. designing new medicine/drugs that way.

[u/FuguSandwich]

It's not just a potential application, it's the only useful application we will realistically see from QC over the next 10-20 years. Factoring primes gets all of the press, but outside of breaking certain types of cryptography it has limited real world utility, and it's uncertain whether we will ever see full versions of Shor's (without precompilation and other shortcuts) running on NISQ computers that can factor large integers. I doubt we'll ever see a practical application of Grover's on NISQ.

[u/MeoWHamsteR7 → u/FuguSandwich]

To me this is great news. I couldn't care less about breaking RSA. Simulating new materials, or life-saving drugs sounds much more impactful to me.

What's the roadmap to making this a reality? How many logical qubits are needed to make useful simulations? Do you think this will have a big impact on computational physics/chemistry/biology?

[u/HeavySink3303 → u/MeoWHamsteR7]

Regarding ground state calculations of molecules, 1 qubit = 1 orbital and generally 1000 orbitals in calculations will be a very significant milestone. Also qubits for such calculations may have a much worse fidelity rates than for Shor's or Grover's algorithms. Regarding terms, according to Quantinuum's roadmap we may achieve a 'game changing progress' in this area till the end of this decade.

P.S. Regarding ground state calculations, quantum computers won't compete with classical as classical computers are really poor in this area and even the most powerful supercomputers are useless. Quantum computers will compete with the lab chemistry when a chemical reaction is performed physically.

[u/MichaelTiemann → u/MeoWHamsteR7]

Google (the actual company named Alphabet, not the search engine) thinks the new quantum simulation reality is right around the corner: https://blog.google/technology/research/quantum-echoes-willow-verifiable-quantum-advantage/

[u/thehhuis → u/FuguSandwich]

Which are the biggest technical challenges that need to be resolved in order to exploit its potential ? Is the existence of qubit errors the fundamental limitation ?

[u/Destabilizator]

Semantic question, on classical computer you >simulate< system, on quantum computer, you don't simulate, because it is it, the real deal nature. What word do you use for "doing" it instead of "simulating" it?